Spread spectrum technology, or spreading technology, is a technology developed very quickly in the recent years, which not only plays an advantageous role in military communication but also is widely used in every field of communication, for example, satellite communication, mobile communication, microwave communication, wireless positioning system, wireless local area network (LAN), global personal communication.
Spreading communication can be briefly expressed as follows: a way for transmitting information, with the signal occupying a bandwidth far more wider than the narrowest bandwidth necessary for transmitting information; the bandwidth is spread by encoding and modulation methods and is irrelevant to the transmitted information data; and the transmitted information data are despread and restored by an identical spreading code at a receiver. Spreading technology is used in the following forms: direct sequence spread spectrum (abbreviated as Direct Spread (DS)), Frequency Hopping (FH), Time Hopping (TH) and linear frequency modulation (Chirp), and the combinations thereof such as FH/DS, TH/DS and FH/TH, wherein DS, FH and FH/DS are used most frequently in the communication.
Spreading communication technology is characterized as follows:
1. Potent Anti-Scrambling Capacity
Since signal is spread to a wide band, narrowband signal is restored by compressing bandwidth of spread signal at receiver. Because the scrambling signal, which is irrelevant to a spreading pseudo-random code, is spread to a wide band, the scrambling power, which enters into communication bandwidth of a useful signal, is greatly reduced, and then the signal-to-interference ratio of the correlator is increased. Thus the spreading communication technology has a strong anti-scrambling capacity. This capability is in a direct ratio to the spreading times of the band: the wider the spectrum is spread, the stronger anti-scrambling capacity is.
2. Multiple Access Communication
Spreading communication per se is a way of multiple access communication, named SSMA (Spread Spectrum Multiple Access). It per se is a form of CDMA (Code Division Multiple Access), which forms different networks with different spreading codes. Although occupying a wide band, the spectrum utilization ratio of a spreading system is higher than that of a single channel single carrier system, because different networks can share the same frequency simultaneously. CDMA is a main form of multiple access communication for the future global personal communication.
3. Secure and Confidential
Since spreading system spreads the transmitted information to a wide band, the power density reduces with the spreading of the spectrum, and the signal may even be submerged by the noises. Accordingly, the spread signal has strong security, and is quite difficult to be intercepted, eavesdropped and detected. Unless the same spreading code as that used by transmitter is used and correlation detection is performed after synchronization therewith, nothing can be done about the spread signal. Since the spread signal has a low power spectrum density, a specific frequency band, e.g. ISM frequency band, can be used without approval in many countries such as USA, Japan and European countries, as long as the power spectrum density meets some requirements.
4. Multi-Path Interference Rejection
Under some communication circumstances such as mobile communication and indoor communication where there is serious multi-path interference, a system must have a strong anti-interference capacity to ensure unimpeded communication. Spreading technology has a strong multi-path interference rejection capacity by use of the relevant characteristics of the spreading code used by spreading, and can even improve the performance of the system by use of multi-path energy.
CDMA system, which employs spreading technology, operates quite differently from the conventional TDMA and FDMA communication systems. In a direct sequence-CDMA (DS-CDMA) transmitter, for example, a digital symbol stream for a given dedicated or common channel at a basic symbol rate is spread to a chip rate. This spreading operation involves applying a channel unique spreading code (sometimes referred to as a signature sequence) to the symbol stream that increases its signal gain while adding bandwidth redundancy.
Typically, the digital symbol stream is multiplied by a channel code during spreading to derive a channelized intermediate signal including data information. The intermediate signal is then added to other intermediate signals using different channel codes. The summed intermediate signals are then multiplied by cell scrambling code to obtain a group of spread signals. Since all channel codes are orthogonal with each other, different users may share transmitting bandwidth at a specific time slot via different channel codes. And through the application of proper processing techniques at the receiver, the intermediate signals may be distinguished from others.
In the TD-SCDMA systems, the original data are restored by applying (i.e. multiplying, or matching) the appropriate scrambling codes and channel codes to despread spread signal at the receiver. Where the spreading code is applied to other transmitted and received intermediate signals, however, only noise is produced. The despreading operation thus effectively comprises a correlation process that compares the received signal with the appropriate digital code to recover the desired information from the channel.
In the TD-SCDMA systems, spreading is applied to the data part of the physical channels and comprises two operations. The first is the channelisation operation, which transforms every data symbol into a number of chips, thus increasing the bandwidth of the signal. The number of chips per data symbol is called the Spreading Factor (SF). The second operation is the scrambling operation, where a scrambling code is applied to the spread signal. The two operations are generally called spread operation together, wherein channel code and scrambling code are called spreading code together. It is required in TD-SCDMA system standard that downlink physical channels shall use SF=16. Multiple parallel physical channels can be used to support higher data rates. These parallel physical channels shall be transmitted using different channelisation codes.
The spread and despread of signals can be expressed by the following formula.
                                          z            ⁡                          (              i              )                                =                                    x              ⁡                              (                                  ⌈                                                            [                                                                        (                                                      i                            -                            1                                                    )                                                /                        SF                                            ]                                        +                    1                                    ⌉                                )                                      ⁢            p            ⁢                          {                                                [                                                            (                                              i                        -                        1                                            )                                        ⁢                                                                                  ⁢                    mod                    ⁢                                                                                  ⁢                    SF                                    ]                                +                1                            }                                      ⁢                                  ⁢                              x            ⁡                          (              n              )                                =                                    1              SF                        ⁢                                          ∑                                  i                  =                  1                                SF                            ⁢                                                z                  ⁡                                      [                                                                                            (                                                      n                            -                            1                                                    )                                                ×                        SF                                            +                      i                                        ]                                                  ⁢                                                                            p                      *                                        ⁡                                          (                      i                      )                                                        .                                                                                        (        1        )            
Where x(n) is the signal before spreading, p(i) is the spreading code and z(i) is the signal after spreading, ┌A┐ denotes the round operation and the return value is the nearest integers greater than or equal to A.
The conventional despreading method and apparatus require a large amount of multipliers. However, such multipliers have complicated structures and occupy a relatively large chip area. So, if they can be replaced by other simple circuit structure, then the chip area can be greatly reduced and the production cost decreased by a big margin. Therefore, there is a need for a despreading method and apparatus capable of reducing the production cost significantly.